INTRODUCTION. Feies gave the name of Myxogastres, in 1833, to the group of organisms described in this Monograph, placing it among the Gasteromycetous Fungi. In 1836 Wallroth substituted the term Myxomycetes (Schleimpilze) for the older name, and this came to be the generally accepted designation. Later investigations showed that the spores, instead of producing a mycelium, as in tjie case of fungi, gave birth to swarm-cells, which coalesce to form a plAsmo- dium. In consequence of this discovery, which indicated a relation¬ ship with the lower forms of animal life, de Bary in 1858 introduced the name Mycetozoa. Under this head he still retained the term Myxomycetes for the section so named by Wallroth, but linked with them the Acrasiece of Van Tieghem, a small group inhabiting the excrement of animals; in these the spores are said to produce swarm-cells, as in the Myxomycetes, which multiply by division but do not coalesce to form a plasmodium. At a certain period, when the fruits are about to be formed, they become attached in branching strings which concentrate to a point, where they are massed together in aggregations of more or less definite shape ; the swarm-cells, however, do not lose' their individuality. In Dictyostelium, a genus of the Acrasiece, a stalk is formed by the arrangement of a number of swarm-cells in vertical rows iu the centre of the heap; the surrounding amceboid bodies creep up this stalk and form a globose cluster at the extremity; here each amoeboid swarm-cell acquires a spore-wall, and they become a naked aggregation of spores not enclosed by a definite sporangium- wall. Eostafinski followed de Bary in the view that the formation of a Plasmodium indicates, a wide separation in the natural position of the Myxomycetes. from the fungi, but he suppressed that name entirely, adopting de Bary's class name Mycetozoa in its. place; at the same time, he admitted into his MonographDictyostelium, a genus of the Acrasiece. The reason for his includingthis genus may be the fact pointed out by de Bary, that Brefeldin first describing the dense aggregations of swarm-cells into thestalked spore-masses of Dictyostelium,, refers to them as being " Plasmodia; that is, products of the coalescence of swarm-cells ;" and it was not until after the publication of Rostafinski's 1